The invention relates to a semiconductor diode laser--often referred to as laser for short hereinafter--with a semiconductor body comprising a substrate of a first conductivity type and situated thereon a semiconductor layer structure with at least a first cladding layer of the first conductivity type, a second cladding layer of a second conductivity type opposed to the first, and between the first and second cladding layers an active layer and a pn junction which, given a sufficient current strength in the forward direction, is capable of generating monochromatic coherent electromagnetic radiation in a strip-shaped active region situated within a resonance cavity formed between two end faces, the active layer comprising one or several quantum well layers of a first semiconductor material which are mutually separated or surrounded by barrier layers of a second semiconductor material, while the second cladding layer and the substrate are electrically connected to connection conductors and a first portion of the quantum well and barrier layers forming part of the active layer has a compression stress because the semiconductor material in the first portion has a lattice constant which is greater than that of the substrate, and a second portion of said layers has a tensile stress because the semiconductor material in the second portion has a lattice constant which is smaller than that of the substrate. It is noted that the term "barrier layers" also refers to so-called separate confinement layers. The invention also relates to a method of manufacturing such a laser.
Such a laser is known from the U.S. Pat. published under No. 5,373,166 on Dec. 13th, 1994. The laser disclosed therein is manufactured in the InP/InGaAsP material system which corresponds to the wavelength region from 1 to 1.5 .mu.m, and utilizes a Multi Quantum Well (MQW) active layer in which the quantum well and barrier layers comprise a material with a lattice constant which is alternately greater than and smaller than that of the substrate. A greater lattice constant results in a compression stress of the relevant layer, a smaller in a tensile stress. Such a laser has a starting current which is much lower than that of a laser in which all layers exactly match the substrate owing to the influence of the stresses on the shape and position of the valency and conduction band. The known laser is stress-compensated, i.e. the total compression stress is approximately equal to the total tensile stress. In other words, the product of the absolute value of the relative difference in lattice constant and the thickness is the same for both types of layers. Defects in the active layer of the laser will occur less readily thanks to this stress compensation, which benefits useful product life. In addition, such a laser also has a strongly reduced starting current.
A disadvantage of the known laser is that it has too short a life, especially if the laser is manufactured in the GaAs/AlGaAs or the InGaP/InAlGaP material system. This is sometimes a disadvantage in the application in an optical disc system, laser printer, or bar code reader, especially if a high power level is desired there.